1. Field of the Invention
The present invention generally relates to the field of gas sensing devices. More particularly, the present invention relates to gas detectors capable of measuring the concentrations of one or more gases using a characteristic infrared absorption band of the gas to be detected.
2. Description of the Prior Art
Many gases have characteristic absorption bands falling within the infrared spectrum. The nondispersive infrared (NDIR) technique has been widely used in the gas analyzer industry for the detection of these gases. Such gas analyzers utilize the principle that various gases exhibit substantial absorption at characteristic wavelengths in the infrared radiation spectrum. Typically, a narrow-band optical or infrared transmission filter is used to isolate the wavelength band of interest in NDIR gas analyzers. On the other hand, a prism or diffraction grating is used in gas analyzers relying on dispersive techniques.
The NDIR technique, which is generally classified as a non-interactive gas analysis technique, offers a number of advantages over previous interactive types of gas measurement methods including electrochemical fuel cell, sintered semiconductor (tin dioxide), catalytic (platinum bead) and thermal conductivity. These advantages include speed of response, gas detection specificity, long term measurement stability, reduced maintenance, and greater specificity. Moreover, in some cases the interactive gas sensor can be poisoned into a nonfunctional state. Depending on the application, this could place human life at risk.
Interactive gas sensors are generally nonspecific because the reagent being used to determine the concentration of the desired gas may react with other gases that are present. This will naturally result in false readings. Further, if the equilibrium of the reaction between the nonspecific gas and the reagent is such that the gas and reagent remain reacted even after the partial pressure of the gas drops in the environment being monitored, the sensor will no longer function properly and is poisoned.
The response time for NDIR gas sensors is typically shorter than that for interactive gas sensors because the kinetics of the reaction between the sample gas and reagent controls how quickly the reactive type sensor can detect a change in the concentration of the gas in the environment being monitored.
Despite the fact that interactive gas sensors are unreliable and that the NDIR gas measurement technique is one of the best, NDIR gas analyzers have not enjoyed wide spread application because of their complexity and high cost of implementation.
Over the years, a large number of measurement techniques based upon the NDIR principle for the detection of gases have been proposed and successfully demonstrated. In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source onto the detector.
The most notable of these types of analyzers are shown and described in U.S. Pat. No. 3,793,525 to Burch, et al., U.S. Pat. No. 3,811,776 to Blau, Jr., and U.S. Pat. No. 4,578,762 to Wong. These NDIR gas analyzers perform well functionally and have contributed greatly to the overall technical advancement in the field of gas analysis during the past two decades. However, their overall size, complexity, and cost have precluded their use in a number of applications.
The need for better and lower cost gas analyzers has led to newer inventions. For example, U.S. Pat. No. 4,500,207 to Maiden and U.S. Pat. Nos. 4,694,173 and 5,026,992 to Wong have proposed NDIR techniques for gas detection that do not use any moving parts such as mechanical choppers. The goal of these patents has been to produce NDIR gas sensors that are more rugged and compact, thus opening up a host of new applications.
In an attempt to further reduce the cost and simplify the implementation of the NDIR technique, a low-cost NDIR gas sensor technique was developed. The low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued in Nov. 17, 1992, to the present applicant, and hereby incorporated by reference. This diffusion-type gas sample chamber eliminates the need for: expensive optics, mechanical choppers, and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications for the NDIR technique, which were previously considered impractical because of cost and complexity, have been opened.
A similar guiding principle led to the development of the improved NDIR gas sensor disclosed by Wong in U.S. Pat. No. 5,444,249. This patent describes a simple, low-cost diffusion-type NDIR gas sensor which can be micromachined out of a semiconductor material such as Si or GaAs, thus allowing the entire sensor to be placed on a microchip.
Although the low-cost NDIR gas sensor technique of U.S. Pat. No. 5,163,332 and the improved NDIR gas sensor of U.S. Pat. No. 5,444,249 have opened a wide variety of new applications, these gas sensors still require too much power to be used in many potential gas sensor applications. As a result, applications in which low-cost, solid-state gas sensors may be used remain limited.
If a gas analysis technique could be developed which required no moving parts, had the same degree of specificity as the NDIR technique, was low cost, and had relatively low power demands so that devices employing the technique could be battery operated over an extended period of time, the applications in which gas sensors are used and the frequency of their use would increase dramatically. Therefore, while a long felt need exists for a simple, compact, inexpensive gas sensor that has low power requirements, this need has gone unfilled. Accordingly, a goal of the present invention is to further advance the technique of infrared gas analysis by providing a compact, reliable, low cost, and low power infrared gas sensor using infrared absorption.